Saturday, March 18, 2006

Externalist a priori justification

Something is a priori if it can be justified independently of experience. Dave Chalmers connects the a priori to a deep sense of "epistemic necessity". (We might roughly characterize this as what an ideal agent could recognize as being guaranteed to be true, no matter how the actual world turns out.) This connection would be problematic if there were a posteriori certainties. If I know something for certain, to deny that it is thereby "epistemically necessary" arguably stretches the phrase beyond all recognition.

But Chalmers holds (plausibly enough) that introspection is a form of experience. So the paradigmatic example of certainty -- the conclusion of Descartes' cogito ("I think, therefore I am") -- is a posteriori, and hence "epistemically contingent" on the above definition.

Externalists about justification can avoid this conclusion, however. My belief that I exist is as safe as safe can be. There is no possible world in which I am mistaken in having this belief. (Merely having the belief suffices to ensure that I exist, and thus that the belief is true!) And note that introspective experience plays no role in this externalist justification. It suffices that, as a matter of objective fact, I couldn't possibly be wrong. Since similar considerations will apply to any purported "a posteriori certainty", we see that they can be justified independently of experience -- and thus qualify as a priori -- after all.

Update: On second thought, I think I'm wrong about that. Consider the thought "I am conscious". I take this to be an a posteriori certainty. Given my introspective evidence, I couldn't possibly be wrong. But there are zombie worlds in which my counterpart is mistaken in having this belief. Of course, he'll lack the introspective evidence that I have, since he isn't conscious. But that's a fundamentally a posteriori difference. It seems that, even for externalists, there's no way to account for this epistemic necessity in purely a priori terms. We cannot vindicate a connection between the a priori and this sense of "epistemic necessity".

[Update II: Oops, I forgot that Chalmers characterizes his "deep" sense of epistemic necessity as being unrelativized to what anyone actually happens to know. It is instead supposed to underlie our more particularized or "strict" sense of epistemic necessity (which need not be tied to the a priori). So my earlier criticisms were thoroughly misguided. That doesn't much matter for the more general discussion of externalism, however.]

A 'zombie world' is a world physically identical to our own but lacking in phenomenal consciousness or qualia. It is filled with people just like us, except that they are not truly conscious -- there is not "something it is like" to be them. Philosophers call those people 'zombies'. Now, when my zombie twin says "I am conscious", he says something false.

Another way to make the point is as follows: If (counterfactually) I hadn't been conscious, then I still would have believed myself to be conscious (at least in the 'functional', deflationary, sense of 'belief'), and that belief would have been false. So my belief that I am conscious is not absolutely "safe" in the externalist's sense. I could have had the belief mistakenly. (It is only certain in light of my introspective evidence. It would not be certain without that, i.e. a priori.)

But then could a Zombie be thinking "I am conscious"? He could say it, but could he think it in the sense Descartes considers? It doesn't seem like he could since the thought requires consciousness of a sort the Zombie lacks.

Does that matter, though? Or is the existence of mistaken deflationary/functional states sufficient to trouble the externalist, even if there aren't any conscious states which go wrong here? Perhaps if we think zombies aren't really persons at all, then my zombie-twin is not a genuine counterpart of mine, and so can be excluded from consideration. This might allow the externalist to deny there are any a posteriori certainties after all. (But it seems kind of wrong-headed to me. I'll have to think about it some more.)

G, did you mean to point to the tautology: "if I am typing, then I am typing" (as a specific instance of the form, "if P then P")? Such trivial logical truths can clearly be known a priori, without appeal to experience, so they're not interesting cases for the present discussion. I'm wondering whether there are things we can know with certainty, but only on the basis of experience. Rationality alone is not enough to justify them. "I am conscious" seems quite different from mere tautologies in this respect. [If that isn't what you meant, note that you cannot know with certainty that you are typing. You might just be hallucinating the whole experience.]

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